In the fields of electronics and microelectronics, there is an ever present desire and need for materials with improved electrical characteristics. Design trends in microelectronics have led to the use of larger integrated circuit chips. Additionally, the density of circuitry on the chips has increased the power dissipated by the chips. These trends have been limited by the available electronic packaging used to house the chips and have led to a demand for improved electronic packaging.
The improved performance demanded of electronic packaging (and the materials systems making up the packaging) is multi-faceted. The package should be able to dissipate heat generated by the chips contained therein. The package should allow transmission by multiple electrical circuits between the chips to the package exterior. The package should allow transmission of high speed electrical signals within and between the chips held in the package. The package should have good thermal expansion match with the chips (usually silicon-based) to withstand thermal cycling without excessive stress buildup. The package should be able to adequately protect the chips from the outside environment. Further, the package should be able to maintain its own performance in the operating environment over time.
Two properties of the package which limit I.C. chip design are the thermal conductivity and the dielectric constant of the package. Larger, higher density, higher speed I.C. designs use more power and generate a greater amount of ohmic heat. I.C. performance degrades at high temperatures. Therefore, improved ability of the package to dissipate heat is essential to the reliable operation of high power designs. The packages ability to dissipate heat is directly related to the thermal conductivity of materials forming the package as well as the package design.
Low dielectric constant materials are particularly important with regard to the speed of electrical signal propagation for the I.C. chips in the package. Lower dielectric constant materials allow higher propagation speeds. Higher propagation speed is very desirable since it effectively increases the capability of the chips to process signals more quickly.
In the past, various materials systems and package designs have been employed to meet electronic packaging demands. Typically, those packages either had good thermal conductivity or low dielectric constant, but not both.
In U.S. Pat. No. 4,865,875 to Kellerman, a composition containing organic binder, ceramic or glass ceramic particles, and a plurality of hollow glass microspheres was applied as a thick film on an alumina substrate. The composition was subsequently fired to yield a material having a dielectric constant of about 3.5-4.5. While this package has a low dielectric constant material, the package has poor thermal conductivity. Additionally, the thick film process used in Kellerman is potentially costly and difficult to control.
U.S. Pat. No. 4,867,935 to Morrison discloses the formation of multilayer structures from green sheets containing glass microspheres and glass particles. The green sheets are pressureless sintered to form a hermetic body. Morrison uses this low dielectric constant composite material to form the entire package base. The lowest dielectric constant reported by Morrison was 3.25 at 1 MHz using a 40 vol. % loading of microspheres. The Morrison package unfortunately also has very poor thermal conductivity.
Thus, there remains a need for improved electronic packaging having both low dielectric constant and high thermal conductivity.